In recent years, welded steel pipes used for transporting natural gas and crude oil have been required to achieve enhancement of transport efficiency by using a higher pressure and enhancement of efficiency of performing on-site welding by decreasing the wall thickness of the pipes. Accordingly, welded steel pipes having a higher strength and a larger wall thickness are provided year by year.
In addition, since steel pipes have come to be used in cold regions where ground deformation occurs, low-temperature toughness of weld zones and buckling resistance are required to be enhanced and there is a demand for the development of a X100-grade thick-wall steel pipe satisfying such requirements.
In chemical composition design of high strength steel plates used for X100-grade steel pipes, addition of B is effective to achieve sufficiently high strength and toughness. However, in the case of steel pipes, it is also important to satisfy welding properties such as cold cracking susceptibility. Accordingly, in chemical composition design of X100-grade steel pipes, to prevent cold cracking in circumferential weld zones that are formed by low heat input welding and connect steel pipes together, a chemical composition design in which boron (B) having high hardenability is not added to base steel plates has been basically used (for example, NKK Technical Review No. 138 (1992), pp. 24-31 and NKK Technical Review No. 66 (1992)).
However, it is reported that, with an increase in the strength of steel plates, addition of B results in excellent toughness of seam-weld heat affected zones depending on welding heat input to seam weld zones (for example, Journal of Japan Welding Society No. 50 (1981)). Japanese Unexamined Patent Application Publication No. 2006-328523 discloses that, in seam weld zones of steel pipes, diffusion of B contained in weld metal into base material results in enhancement of toughness of seam-weld heat affected zones near fusion lines.
In welded heat affected zones of B-added high strength steel, even when a prior austenite grain size in regions relatively away from fusion lines is a small size of 150 μm or less, there are cases where an upper bainite structure containing a large amount of island martensite (also referred to as MA: Martensite-Austenite Constituent), which is detrimental to toughness, becomes dominant and toughness is degraded. Thus, in high strength steel, the influence of addition of B on the toughness of welded heat affected zones is not sufficiently understood.
In chemical composition design of an X100-grade steel pipe having a large wall thickness of more than 20 mm, to ensure strength, toughness, deformability, and circumferential weldability and to ensure excellent low-temperature toughness of welded heat affected zones in seam weld zones, the influence of addition of B on the structure of welded heat affected zones has been intensively studied.
Japanese Unexamined Patent Application Publication Nos. 2008-56961, 2004-131799, 2003-306749 and 2003-293078 relate to high strength welded steel pipes and production methods of high strength welded steel pipes and state that, when B is added to base material compositions, an appropriate amount of B is added in consideration of the toughness of welded heat affected zones. In addition, JP '749 and JP '078 propose that, when the alloy amount in base material is made appropriate, different parameter formulae are used depending on whether B is added or not.
Over that period, there are cases where APIX100-grade high strength welded steel pipes are required to have buckling resistance, which is not sufficiently discussed in JP '961, JP '799, JP '749 and JP '078. For example, JP '961 discloses a technique of enhancing the toughness of welded heat affected zones. However, deformability of base material is not studied in JP '961.
JP '799 deals with the X80 grade, which is a strength level different from that discussed in the present invention. JP '749 and JP '078 define the uniform elongation of a base material portion in a tensile test in a pipe axis direction. As described below, it is important to make the ratio (YR:yield ratio) of 0.5% proof strength to tensile strength be low for enhancing buckling resistance, which is not studied in JP '749 and JP '078.
Welded steel pipes used as line pipes such as UOE steel pipes and ERW steel pipes are produced by subjecting steel plates to cold forming into pipes and to welding of abutting portions, and then generally subjecting the external surface of the steel pipe to a coating treatment in view of corrosion resistance and the like. Accordingly, due to strain caused by working in the formation of pipes and heating in the coating treatment, strain ageing is caused and the 0.5% proof strength increases. Thus, a steel pipe having been subjected to the coating treatment has a yield ratio higher than that of steel plates, which is problematic. However, this respect is not solved by the techniques described in Patent Literatures 1 to 5. Accordingly, there is a demand for a high strength welded steel pipe that has a low yield ratio even after the coating treatment and, as a result, has high buckling resistance.
It could therefore be helpful to reveal the influence of addition of B to base steel plates used for APIX100-grade thick-wall steel pipes on weldability and the toughness of welded heat affected zones and provide an APIX100-grade high strength steel pipe for low-temperature usage that is excellent in terms of buckling resistance and toughness of welded heat affected zones, has a wall thickness of 20 mm or more, has base material properties in which the tensile strength is 760 MPa or more and 930 MPa or less, the uniform elongation is 5% or more, and a ratio (YR:yield ratio) of 0.5% proof strength to tensile strength is 85% or less, and has a charpy absorbed energy of 100 J or more in the weld bond at −30° C. In consideration of buckling resistance after a coating treatment, it could also be helpful to provide a high strength welded steel pipe that has strength characteristics and deformability equivalent to those described above even after the steel pipe is subjected to a coating treatment.